Cotton and other cellulose scraps produced when cutting cotton fabrics in clothing manufacture are a waste product often buried in landfills or consumed in incinerators. Garnetting or other maceration techniques to separate and recover the cotton fibers from the scrap shortens the fiber lengths, and the resulting products have few end uses. As a consequence, over 200 million pounds of denim scrap in the U.S. alone is destroyed as waste each year.
U.S. Pat. Nos. 5,376,143 and 5,471,720 describe a process for recycling denim waste by separating the fibers, preparing a colored yarn of a blend of the recycled fibers and virgin fibers, and preparing denim or similarly dyed fabric from the yarn. This process has not been commercially implemented, perhaps because of costs of fiber separation and the limitations of the shortened fibers in making a strong, durable fabric.
Many applications of cotton, however, do not require long fibers. Cotton batting is a popular absorbent because of its softness and cushioning characteristics and high water absorbency. It is a preferred component for many industrial and household products, such as quilts, upholstery, sanitary napkins and diapers, and medical products such as swabs, bandages and the like. However, most of these applications require that the cotton fibers be purified, colorless, and strong, and a process for recycling cotton scrap to produce cotton fibers for these applications has not been commercially feasible because of the difficulties in processing the scrap. One principal area of difficulty is removal and/or decolorizing of the vat and sulfur dyes present in many cotton scraps such as denim.
Vat dyes consist of solubilized colored compounds which are usefully precipitated as the insoluble form within cellulosic fibers. These compounds are reversibly changed to a water-soluble "leuco" state by chemically reducing them in an alkaline reduction process. This is done easily by mixing the dye into a water solution containing a water-soluble reducing agent, such as sodium hydrosulfite, and alkali, such as sodium hydroxide. In a dying process, the cellulosic fiber is typically immersed in a bath containing such a reduced leuco solution, and the dye is allowed to penetrate the substrate.
After this immersion, the fiber is exposed to an oxidizing environment. Such an environment is air and, in one such process, the yarn, wetted in a leuco solution, is draped in long beams over rolls and exposed to air until the dye and accompanying reducing agents are oxidized. Dilute hydrogen peroxide or another peroxygen compound can also be used for this oxidation. In each case, the oxidized medium converts the leuco dye into its original water-insoluble state. If the dye molecule is contained within the cellulose substrate, the water-insoluble dye is trapped and cannot be removed by casual exposure to water and detergents.
Fabric is often dyed with more than one leuco dye. It is common practice to dye dark shades of indigo first with a black sulfur dye and second with blue indigo dye. The term "dye" as used herein is defined to include a single dye or a mixture of dyes.
U.S. Pat. No. 5,366,510 describes a process for desizing and color-fading indigo-dyed garments by contacting them with a reducing agent in an aqueous solution to extract dye materials before the fabrics are bleached to produce the faded, "stonewashed" or "acid-washed" appearance popular with denim materials. The dye is removed in a conventional rotary drum washer-extractor. Treatment of denim scrap in this procedure would tend to unravel the scraps, producing loose yarn, fiber, and tangles of yarn which foul the bath and equipment components. This treatment also causes the denim scraps to roll up into tubes from which it is difficult to extract dye from the innermost layers.
Dye removal and recovery is highly desirable for the economical recovery of useful products from textile scraps such as denim scraps. The value of the recovered dye partially offsets the costs of recycling. Also, the recovery removes an undesirable component from the waste water.
Solvent removal of dyes from synthetic fabrics is a known procedure. U.S. Pat. No. 1,839,819 describes a method for removing dyes from synthetic textiles using heated organic solvents selected to swell the cellulose acetate and cellulose ether fibers and remove solvent-soluble dyes without any chemical change of the dyes, thus preserving the dyes for reuse.
However, an effective solvent extraction system for textile scraps like denim scraps has not been feasible. The solvent removes fully oxidized dye from the fabric but not the size; however, it does remove natural oils and waxes. Solvent evaporation yields a cake of these components from which wax and oil must be removed by extraction with another solvent that does not dissolve the dye.
Separation of indigo dyes from aqueous solutions by oxidation is known. U.S. Pat. No. 126,663 describes an early mechanized system to reduce hand labor in the manufacture of indigo. In this process, indigo plant is fermented to produce an aqueous solution containing the plant extract. The liquid is separated from the residual plant material and agitated in air to reduce the carbon dioxide content and oxidize the indigo materials. The oxidation product forms a solid precipitate, which is then separated from the solution. However, agitating an organic solvent solution in air does not lead to precipitation of the dye components since they are soluble in their fully oxidized form.